Polyethylenes are used to prepare food packaging films, grocery bags, stretch-wraps, shrink-wraps, garbage can liners and shipping bags. Some applications, such as wrapping prepared or raw foodstuffs, require films that provide a sufficiently low water vapor and oxygen transmission rate to preserve the contents.
Packaging films with low water vapor transmission rates (WVTR) and low oxygen transmission rates (OTR) are generally produced by laminating foil, metalized films (e.g., Al on PET or PE), or through the use of metal oxides (e.g. silicon dioxide (SiOx), titanium oxide (TiOx) or aluminum oxide (Al2O3)), which can be deposited by chemical vapor deposition. The addition of metalized films or metal oxides increases the cost and complexity of producing these films. Laminated polyvinylidene chloride (PVdC) films are also commonly used because of their excellent barrier properties and low water vapor and oxygen transmission rates. Although the above mentioned laminated films have low water vapor and oxygen transmission rates these films tend to be more expensive to produce than polyolefin films.
Polyethylene films have also been disclosed. For example, U.S. Pat. No. 6,613,841 discloses the preparation of a machine direction oriented (MDO) polyethylene film that is a blend of a high-molecular weight, medium-density polyethylene (HMW-MDPE) and linear low-density polyethylene (LLDPE). U.S. Pat. Nos. 7,011,892 and 7,078,081 disclose the uniaxial orientation in the machine direction of a high-density polyethylene film (HDPE) that is produced by a high-stalk blown extrusion process. While polyethylene films can provide a sufficient barrier to water vapor due to their hydrophobic nature they generally need to be combined with additional layers or materials to improve their oxygen barrier properties.
A polyethylene film that has the low water vapor and oxygen transmission rates comparable to a PVdC or a laminated polymer film (e.g. metal foils, metal oxides) without the additional complexity or cost would be commercially useful.